In the past, it has been very difficult or impossible to produce small sharp edged apertures in an optically flat polished surface in a very thin section of a substrate without disturbing the optical flatness of the surface.
Typical of the prior art is a process which is very low in yield and quality and in which a substrate is polished on one side to produce an optically flat polished surface and then machined mechanically or by electron discharge machining (EDM) down almost to the polished surface from the opposite side of the substrate with the aperture subsequently being drilled through the remaining material. This machining leaves severe stresses which can damage and distort the polished surface and the thickness of the thin section in which the aperture is formed is limited. Additionally, using EDM causes extremely non-linear material removal and it is unreasonably difficult to achieve an appropriate thinness for the appropriate formation of the apertures.
The production of micro-apertures in an optically flat surface requires very thin unsupported sections which are too thin to be polished in such an unsupported form.